Something else that needs to be taken into consideration, at least in the U.S., is that racial minorities and poorer people are less likely to seek out or participate in mental health services. There is a social taboo. I am not discounting anything that anyone else has said, just adding this fact.
As someone who is a black male and has Asperger's, I have to agree that is it incredibly isolating. I do not fit any black stereotypes, and black people seem to be the people who are the most angry about it. I don't have black friends. The only black people I interact with are my family. All my friends are of some other race. All the women I date are of some other race. This is because I was rejected/cast out of the black community when I was growing up. Everybody hated me because I "acted white." White people usually accept me or at least treat me nicer, even if it they think I am subhuman monkey.
There is another factor that needs to be considered as well though. This is rural/urban. Most black people in the U.S. are middle class. We are normal. We are not what the minstrel show called pop culture displays us as. However, there is a socioeconomic divide within the black community. If you live in a part of the country where there aren't that many blacks, you are less likely to find another black person who has not bought into the minstrel show mentality of blackness. For instance, I did not know that you didn't have to be a "gangsta" in order to be a black male until I went to college because I grew up in such a small, backward, poor town. When I moved to the city, there were all kinds of smart, middle class, professional black men. No one said they were acting white either. They didn't have to choose between their racial identity and their identity as intelligent or studious. To be honest, I am still shocked and somewhat unsettled when I see black people in authority or being treated as equals or I see black kids who are middle class and don't pretend to be poor. It does not fit with the rigid racial hierarchy I was raised with. I like it though. It gives me hope about America.